1. Field of the Invention
The present invention relates to intervertebral disc prostheses for insertion into an intervertebral disc compartment which is formed between a first and a second vertebra. More particularly, the invention relates to intervertebral disc prostheses that restores the natural flexibility of the spine.
2. Description of Related Art
Intervertebral disc prostheses generally comprise two constituent elements each having a supporting plate that rests with an abutment face on an adjacent vertebra when the prosthesis is inserted into the intervertebral disc compartment. The other side of the respective supporting plate supports a joint member that allows a relative movement between the two supporting plates.
U.S. Pat. No. 6,936,071, which corresponds to WO 01/07893 A1, discloses an intervertebral disc prosthesis in which the upper supporting plate comprises a cap-shaped recess. The lower supporting plate is provided with a recessed compartment that receives an exchangeable slide-in insert having a cap-shaped projection. The cap-shaped recess in the upper supporting plate and the cap-shaped projection of the insert form a ball-and-socket joint so that the two supporting plates can be tilted relative to one another in all directions in space.
Before the intervertebral disc prosthesis is inserted into a cervical intervertebral disc compartment, the latter is usually machined in a material-abrading manner. During this procedure bone material is abraded with a milling cutter in order to create appropriately large and plane opposite surfaces on the adjoining vertebrae for the abutment faces of the supporting plates. Fins formed on the abutment faces of the supporting plates prevent slipping of the otherwise plane abutment faces on the adjacent vertebrae. The fins engage grooves which have previously been recessed into the adjoining vertebrae by chiseling. The precise milling of the grooves, however, is a relatively elaborate process. For that reason it is difficult to correctly position the prosthesis in the intervertebral disc compartment.
WO 2005/004756 A1 discloses an intervertebral disc prosthesis that does not comprise fins on the abutment faces. Instead, the abutment faces are slightly convexly curved in a manner that is adapted to the anatomic requirements of the intervertebral disc prosthesis. The geometric data of the healthy intervertebral disc compartment, in particular its height, are determined by extrapolation of data obtained from 3D scanning measurements performed on the diseased spine section. The convex curvature of the abutment faces is said to result in a self-centering effect of the supporting plates within the intervertebral disc compartment. The abutment faces are coated with hydroxyl apatit (HAK) ceramics or TCP materials. These coatings are usually porous and rough which ensures that the supporting plates do not slide within the intervertebral disc compartment.
With such abutment faces it is difficult to obtain both the desired self-centering effect and a durable fixation of the prosthesis in the intervertebral disc compartment. If the abutment faces have a coating with a smooth surface, the supporting plates slide within the intervertebral disc compartment. If the abutment faces have a coating with a rough surface, the friction is too large to obtain a self-centering effect.
EP 0 754 018 B1 discloses a similar intervertebral disc prosthesis. Here the center of motion of the prosthesis, i.e. the center of curvature of the aspherical ball-and-socket joint, is located in the posterior part of the prosthesis, but still between the supporting plates.
WO 03/090648 A1 discloses another prosthesis in which each supporting plate is configured to receive one insert. Each insert is provided with a cap-shaped recess. Both recesses commonly accommodate a ball enabling articulating movement between the supporting plates. The supporting plates have cylindrically curved abutment faces and flat wings arranged at the lateral sides of the supporting plates. Before the prosthesis can be inserted, the adjacent vertebrae have to be prepared by forming elongate recesses in the bone material. The shape of the recesses corresponds to the cylindrical shape of the abutment faces of the supporting plates. In order to prevent sliding movements of the supporting plates relative to the vertebrae along the cylinder axes, the wings are provided with teeth that improve the grip of the wings on the bone material.
Also with this prosthesis its positioning within the intervertebral disc compartment is solely determined by the skill of the surgeon who prepares the cylindrical recesses that accommodate the abutment faces of the supporting plates.